The laying of asphalt paving material on road surfaces entails spreading paving material consisting of an aggregate filled bituminous mixture on a prepared roadbed. The paving material is spread while hot and is then compacted so that upon cooling a hardened pavement surface is formed. Conventional paving machines utilize a heavy assembly termed a “screed” that is drawn behind the paving machine. The screed includes a replaceable screed plate that is constructed of a suitable steel, to spread a smooth even layer of paving material on the prepared roadbed. The weight of the screed assembly aids to compress the paving material and perform initial compaction of the paving material layer. Screed assemblies can include vibratory mechanisms placed directly on the screed plate or separate vibratory tamper bars connected in tandem with the screed plate to aid in the initial compaction of the paving material.
To facilitate laying of the paving material, the screed is typically heated, to a temperature in the range of about 82° to 171° C. (180° to 340° F.). Heating the screed assists the paving material in flowing under the screed and reduces adhesion of the paving material to the screed. If the screed is not adequately heated, the bituminous mixture contacts the bottom of the screed and begins to harden, resulting in buildup of paving material and excessive drag.
Conventional screed assemblies are commonly heated by fossil fuel powered burners that heat the upper surface of the screed plate by the direct application of flame or hot exhaust gases. The use of fossil fuel burners to heat screeds has several drawbacks. Combustion of fossil fuels generates smoke that represents a source of environmental pollution, and also poses a poor working environment for the paving workers. Additionally, because the flames or exhaust gases of the burners actually contact the screed surface, warping may result. The contour of the screed determines the quality, evenness or smoothness of the paving material that is being laid down. Screeds are often flexed under extreme tensile loads during use to achieve desired crowning or other surface contours.
One alternate heating system that represents an improvement in the environmental drawbacks is disclosed in U.S. Pat. No. RE 36,981 issued Dec. 5, 2000 to Ralph Birtchet and assigned to Universal Screed Inc. This patent discloses the use of an elastomeric electrically powered heating pad assembly positioned on the upper surface of the screed with a layer of insulation placed on top of the heating pad assembly. Then, a heavy steel grid member is placed on top of the insulation to hold the heating pad assembly and the insulation in place. The elastomeric material is specifically defined in this patent as being silicone rubber which has poor resistance to tear and abrasion and poor to fair resistance to fluids such as oil, gasoline, and solvents. Additionally, the design requires loose components placed on top of one another to maintain full contact of the heating pad with the screed.
The present invention is directed to overcome one or more of the problems as set forth above.